The hypothetico-deductive model[1], [2] or method is a proposed description of scientific method. It was popularized by Karl Popper. According to it, scientific inquiry proceeds by formulating a hypothesis in a form that could conceivably be falsified by a test on observable data. A test that runs contrary to predictions of the hypothesis is taken as a falsification of the hypothesis. Otherwise the hypothesis is corroborated. It is then proposed to compare the explanatory value of competing hypotheses by testing how stringently they are corroborated by their predictions.

Qualification of corroborating evidence is philosophically problematic. The raven paradox is a famous example. The statement that 'all ravens are black' would appear to be corroborated by observations of only black ravens. However, 'all ravens are black' is logically equivalent to 'all non-black things are not ravens'. 'This is a green tree' is an observation of a non-black thing that is not a raven, and therefore corroborates 'all non-black things are not ravens'. It appears to follow that the observation 'this is a green tree' is corroborating evidence for the hypothesis 'all ravens are black'.

This problem is related to the problem of induction, and arises because a general case (a hypothesis) cannot be logically deduced from any series of specific observations. Since it appears that virtually any observation can be seen as corroboration of any hypothesis, the choice of which observations should be taken seriously seems to be open, rather than a matter of the application of a strict method. The argument has also been taken as showing that both observations and theories are embedded in our overall understanding (holism), and thus it is not possible to make truly independent observations.

Evidence contrary to a hypothesis is also philosophically problematic. Such evidence is called a falsification of the hypothesis. However, under the theory of confirmation holism it is always possible to save a given hypothesis from falsification. This is so because any falsifying observation is embedded in a theoretical background, which can be modified in order to save the hypothesis. Popper anticipated this, noting that the falsification of a hypothesis is a matter of choice on the part of the scientists involved.

Despite these philosophical problems the hypothetico-deductive model remains perhaps the most popular and best understood theory of scientific method.

An algorithmic statement of the hypothetico-deductive method could be stated[3] as

Gather data ( observations about something that is unknown, unexplained, or new )

Hypothesize an explanation for those observations.

Deduce a consequence of that explanation. (A prediction) Formulate an experiment to see if the predicted consequence is observed.

Wait for corroboration. If there is corroboration, go to step 3. If not, the hypothesis is falsified. Go to step 2.